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PHILADELPHIA - LeSean McCoy #25 of the Philadelphia Eagles fumbles the ball that is recovered by K.J. Wright #50 of the Seattle Seahawks during the third quarter of the game at Lincoln Financial Field Dec. 7, 2014. (Photo by Al Bello/Getty Images)

The Eagles gashed the Cowboys’ defense for 464 yards and 33 points on Thanksgiving, but they’ve done that to nearly every team this season. Except two.

The Eagles averaged 3.9 yards per play against San Francisco and 3.3 against Seattle. Against everyone else, they average 5.8 yards per play. So what makes Chip Kelly’s former Pac 12 coaching peers so successful at stopping his offense?

1. Gap, Assignment Discipline

The Eagles have a talented roster, but Chip Kelly is the real star. He’s basically a magician who schemes large running lanes and open wide receivers. He uses unbalanced lines to reposition defenders, read option to hold rushers and brilliant route combinations to create separation.

The No. 1 key to stopping Philadelphia’s offense is simple: Do your job. It sounds so easy, but Kelly’s offense dangles play action and misdirection in defenders’ faces like a carrot on a stick. Plus, they take so little time between plays that defenses’ heads start spinning.

The 49ers’ and Seahawks’ defenses are well coached in this aspect. Edge defenders keep contain, linebackers and safeties read their keys and, most importantly, no one is caught out of position.

The Cowboys defensive front has shown promise when attacking, but the best defense against the Eagles is a “read and react” approach.

In an “attacking” defense, linemen shoot gaps and aggressively swarm to weakness in a formation. The Cowboys linemen, though, must hold the point of attack — “stack and shed” — and diagnose a play before pursuing.

This approach is what announcers often refer to as “bend but don’t break,” as teams sacrifice small amounts of yardage to prevent big plays. Plus, forcing more plays in drives increases the likelihood of mistakes by the offense.

Much like the safe approach by the front seven, playing zone coverage will keep the ball in front of defensive backs, preventing long touchdowns and increasing the chance of interceptions.

The 49ers like to disguise coverage while the Seahawks basically tell offenses how they cover, but their ability in zone coverage is what gives the Eagles trouble. Eagles’ quarterbacks were 1 for 14 on deep passes for 22 yards against the 49ers and 1 for 4 for 35 yards against the Seahawks.

Mark Sanchez seems to like throwing interceptions, and zone coverage gives the Cowboys defensive backs a better chance to help him in that regard.

Outlook

Stopping the Eagle’s offense won’t be as easy for the Cowboys as attacking their defense. The Eagles have a talent advantage in this matchup, so the players will have to be focused and opportunistic.

The good news for Cowboys fans, though, is that the defense doesn’t have to be perfect. As long as the offense has a better showing, which they should, the defense should just have to hold on in the red zone to pull out a win and retake first place in the division.